The invention relates to a method for the self-diagnosis of a particle sensor used for determining a particle content in a gaseous stream, wherein the particle sensor includes on its surface at least two interlocking, interdigital electrodes and a heating element separated from said electrodes by an insulation layer. Said particle sensor can be heated by said heating element in a regeneration phase, and a soot concentration can thereby be removed from the same.
The invention furthermore relates to a device for the self-diagnosis of a corresponding particle sensor, said particle sensor being connected to an engine management system or a sensor control unit. Said engine management system or said sensor control unit includes apparatuses for diagnosing the soot concentration on the particle sensor and the particle sensor itself.
Particle sensors are used today, for example, for monitoring the soot discharge of internal combustion engines and for on-board diagnostics (OBD), for example, for monitoring the operation of particle filters. Collecting and resistive particle sensors are thereby known, which evaluate a change in the electrical characteristics of an interdigital electrode structure due to particle depositions. Provision can be made for two or several electrodes, which preferably interlock in a comb-like manner. The electrodes are short-circuited by an increasing number of particles accumulating on the particle sensor, which results in the electrical resistance decreasing as the particle deposition increases, the impedance decreasing or in a change in a parameter, like a voltage and/or a current, which is related to said resistance or said impedance. A threshold value, for example, of a measurement current between the electrodes is generally defined for the evaluation; and the time up until achieving the threshold value is used as a measurement for the deposited particle quantity. As an alternative, a signal rate of change can also be evaluated during the particle deposition. If the particle sensor is fully loaded with soot, the deposited particles are burned off in a regeneration phase with the aid of a heating element integral with said particle sensor.
A resistive particle sensor of this type is described in the German patent publication DE 101 33 384 A1. The particle sensor is constructed from two interlocking, comb-like electrodes, which are at least partially covered by a retaining shell. If particles from a gaseous stream are deposited on the particle sensor, this then leads to an evaluable change in the impedance of the particle sensor, from which the quantity of deposited particles and consequently the quantity of particles carried along in the exhaust gas can be suggested.
The German patent publication DE 101 49 333 A1 describes a sensor device for measuring the humidity of gases, comprising a resistance measurement structure disposed on a substrate. Said measurement structure interacts thereby with a soot layer and a temperature measuring device is provided. The soot concentration in the exhaust gas of an internal combustion engine can likewise be determined with said sensor device.
A method for controlling the particle deposition on a sensor element, which includes a first electrode and an additional electrode and at which a first voltage U1 as well as a second voltage U2 can be applied at voltage terminals, is known from the German patent publication DE 10 2004 028 997. Provision is thereby made for the sensor element to be able to be operated with an increased voltage U1 during a first time period t1 and after exceeding a triggering threshold AP of the sensor element for said sensor element to be able to be operated at a lower voltage U2, which is lower than the increased voltage U1. The method makes it possible for the time after a regeneration of the sensor element, in which no measurement signal is available, up until the point in time, whereat an evaluable signal is received as a result of a deposition of a sufficient quantity of particles, to be shortened by operating the sensor element with an increased operating voltage during this phase. The increased operating voltage leads to an increased rate of deposition of the particles on the sensor element. If a sufficiently large quantity of particles has been deposited on the sensor element, thereby enabling a usable measurement signal to be present, the sensor element is operated with a lower voltage having a correspondingly lower particle deposition rate so that the measuring time until the next necessary regeneration of the sensor element is lengthened. The method thus provides for two consecutive operating phases, a first phase with increased operating voltage, during which a sufficient measurement signal is still not present, and a second phase with reduced voltage, during which the actual measurement of the particle concentration takes place. In the process, the resistance or the impedance of the sensor element is determined during both phases via an appropriate current measurement, in the one instance to recognize the triggering threshold and in the other to determine the particle deposition rate. A defined particle deposition is necessary in both phases. The selected voltages thus represent in both phases a compromise between optimized particle deposition and a precise resistance or impedance measurement.
A sensor is known from the German patent publication DE 103 19 664 A1 for the detection of particles in a gaseous stream, particularly of soot particles in an exhaust gas stream, which is disposed on a substrate made of an electrically insulating material. Provision is thereby made for the measuring electrodes to be coated with a protective layer. The electrodes are protected from corrosion at harsh ambient temperatures by said protective layer. Said protective layer can thereby be implemented in an electrically conductive manner or as an electrical insulator. A conductive protective layer allows for the particle concentration to be determined by means of a resistive direct current measurement, a parallel connection between the electrodes resulting via the protective layer and the deposited particles. When an insulating protective layer is implemented, an impedance measurement with the aid of AC voltage is required.
In order to regenerate the particle sensor after particle deposition has taken place, the particle deposits have to be burned off said sensor with the aid of an integrated heating element. This process has to be implemented at specific time intervals in order to avoid discrepancies when determining the particle concentration.
In an additional application with the internal file number R.318399, the applicant, for example, provides for the self-diagnosis by having the particle sensor include an additional flat test electrode. Provision is then made in multiple procedural steps for different test voltages to be applied between the measuring electrodes and the test electrode and in each case for the current, respectively capacitance, to be measured and using the ascertained values for an inference to be made as to whether the particle sensor is functioning properly.
The European patent publication EP 1 925 926 A1 describes a device and an evaluation process for checking the functional capability, respectively for checking the plausibility, of a sensor based on an electrode system, in particular a particle sensor, the device comprising at least one reference electrode system. The evaluation process thereby provides for comparative measurements at the measuring electrodes as well as at the reference electrodes, a proper functioning of the sensor being suggested on the basis of the ascertained values.
Because the particle sensor is disposed downstream of a particle filter in the exhaust gas stream when used for on-board diagnostics, particles, in particular soot particles, which could yield a corresponding sensor signal, should no longer be found in the exhaust gas at that location, whereat the particle filter is disposed, when said particle filter is fully operational. The fact that the sensor does not deliver a signal can, however, also mean that the particle sensor is defective, and a particle filter, which is likewise possibly defective, is not recognized as being defective.
It is therefore the aim of the invention to provide a method, which allows for a reliable self-diagnosis of the particle filter, particularly when installed in the aforementioned position.
It is furthermore the aim of the invention to provide an appropriate device for carrying out the method.